As shown FIG. 5(a) and FIG. 5(b), a speed reduction mechanism including a worm 51 and a worm wheel 52 is conventionally proposed. In such a speed reduction mechanism, pressure angles of the worm 51 and the worm wheel 52 are typically set to the same value (e.g., α°) with respect to each other. Patent document 1 discloses a speed reduction mechanism for reducing a wear volume by setting the pressure angle of the worm greater than the pressure angle of the worm wheel.